The present invention relates generally to a process for removing sulfur from a feed stream using a selective membrane and, more particularly, to a sulfur removal process, wherein sulfur species are withdrawn from a hydrocarbon feed stream by selective permeation through a solid membrane.
Environmental regulations have become increasingly strict with respect to the level of sulfur permitted in hydrocarbon products from refineries, gas plants and other sources. As the maximum allowable amount of sulfur in hydrocarbon products shrinks due to tightening governmental constraints, conventional processes for removing sulfur from hydrocarbon streams, such as hydrotreating, become more difficult and costly to perform. Furthermore, many conventional sulfur removal processes are performed under relatively severe temperature, pressure or other process conditions, which can diminish the desirable properties of the treated hydrocarbon. For example, gasoline typically exhibits octane loss when subjected to hydrotreating. The present invention recognizes a need for a hydrocarbon desulfurization process, which is both operationally practical and cost effective, yet which does not substantially diminish the desirable properties of the resulting hydrocarbon product.
Accordingly, it is an object of the present invention to provide a process for effectively removing sulfur from a hydrocarbon. More particularly, it is an object of the present invention to provide a process for effectively removing sulfur from a liquid hydrocarbon stream in a refinery or other hydrocarbon processing facility. It is another object of the present invention to provide a desulfurization process, wherein the sulfur in the liquid hydrocarbon stream is concentrated during a preliminary stage into a smaller stream than the liquid hydrocarbon stream to reduce the throughput to the desulfurization process. It is a further object of the present invention to provide a desulfurization process, wherein the liquid hydrocarbon stream is not subjected to severe process conditions, which could diminish the desirable properties of the liquid hydrocarbon stream. These objects and others are achieved in accordance with the invention described hereafter.
The present invention is a process for removing sulfur from a hydrocarbon. A relatively larger quantity of a feed stream is conveyed past a first side of a solid membrane, while a relatively smaller quantity of a sweep stream is conveyed past a second side of the membrane. The weight ratio of the sweep stream to the feed stream is preferably below about 0.2. The feed stream comprises a feed liquid hydrocarbon and a feed sulfur species, which the practitioner desires to remove from the feed liquid hydrocarbon. The feed sulfur species is substantially more membrane permeable than said feed liquid hydrocarbon. The feed liquid hydrocarbon is preferably a conventional refinery hydrocarbon stream such as naphtha, diesel, or a mixture thereof. The feed sulfur species is preferably an organic sulfur compound, elemental sulfur, hydrogen sulfide, or a mixture thereof. Preferred organic sulfur compounds include thiols, alkylated thiols, thiophenes, alkylated thiophenes, benzothiophenes, alkylated benzothiophenes, dibenzothiophenes, alkylated dibenzothiophenes, and mixtures thereof. The sweep stream comprises a sweep fluid, which is preferably sweep liquid hydrocarbon selected from the group consisting of naphtha, diesel, cycle oil, and mixtures thereof.
The membrane is preferably formed from a solid material, which is relatively more selective for the feed sulfur species than the feed liquid hydrocarbon. Preferred sulfur-selective compounds include nitrogen compounds, nitrogen oxide compounds, oxygen compounds, sulfur compounds, and sulfur oxide compounds, and mixtures thereof. Alternatively, the solid membrane may contain a facilitated transport liquid, preferably an amine, hydroxyamine, alcohol, or mixture thereof. If the solid membrane contains a facilitated transport liquid, the sweep stream preferably includes a decoupling agent species. The decoupling agent species is selected from the group consisting of amines, hydroxyamines, alcohols, sulfur compounds, and mixtures thereof.
The feed sulfur species is transported in a permeate from the feed stream through the solid membrane to the sweep stream. As a result, the feed stream is converted to a relatively larger quantity of a substantially sulfur-free reject stream containing a primary hydrocarbon product, while the sweep stream combines with the permeate to produce a relatively smaller quantity of a sulfur-enriched stream, which is amenable to further processing.
Further processing may comprise distilling the sulfur-enriched stream to separate the feed sulfur species from the sweep liquid hydrocarbon. After separation, the feed sulfur species is dehydrogenated to produce a residual hydrocarbon product, which is combined with the primary hydrocarbon product to obtain a recovered hydrocarbon stream. The sweep liquid hydrocarbon is recycled to the second side of the solid membrane. Alternatively, the entire sulfur-enriched stream is dehydrogenated to produce the residual hydrocarbon product. The residual hydrocarbon product is then separated from the sweep liquid hydrocarbon and combined with the primary hydrocarbon product, while recycling the sweep liquid hydrocarbon.
In accordance with another embodiment of the present sulfur removal process, the sweep stream is omitted from the second side of the membrane. The feed stream is conveyed past the first side of the membrane as above, but when the permeate passes through the membrane to the second side, it is collected and hydrogenated by itself to produce the residual hydrocarbon product.
The invention will be further understood from the accompanying drawings and description.